1. Field of the Invention
The present invention relates to an emission control apparatus that controls the intensity of laser light emitted from a semiconductor laser serving, for example, as a light source for writing image information, and an image forming apparatus such as electrophotographic laser printer, copying machine, or laser facsimile, which includes the emission control apparatus.
2. Description of the Related Art
FIG. 6 is a block diagram showing the general construction of a control system in an electrophotographic laser printer including a conventional laser control apparatus.
In the control system in FIG. 6, a laser beam emitted forward (to the right in FIG. 6) from a semiconductor laser diode 301 is collimated by a collimator lens 302, and deviated by a polariscope 303 comprised of a rotary polygon mirror to be irradiated onto a photosensitive drum 305 via an fθ lens 304. The photosensitive drum 305 has a surface thereof evenly electrified in advance by an electrifier. The photosensitive drum 305 rotates in response to forward and backward scanning of image forming spots by the rotation of the rotary polygon mirror in the direction of the rotational axis (axial direction) of the photosensitive drum 305, so that the entire surface of an image forming region on the photosensitive drum 305 is scanned. It should be noted that in the forward and backward scanning of the photosensitive drum in the axial direction thereof, the laser beam scans not only the image forming region but also marginal regions (non-image forming regions) provided at both sides of the image forming region on the photosensitive drum 305.
A photodetector 306 is provided at one point on the scanning line of the laser beam and outside the image forming region, and detects the laser beam deviated by the rotary polygon mirror in every scanning to generate a BD signal. According to the BD signal transmitted from the photodetector 306, a signal processing circuit 307 controls timing in which an image signal is applied to a semiconductor laser control circuit 308 of a laser control apparatus 300.
The semiconductor laser control circuit 308 turns on/off the semiconductor laser diode 301 according to the image signal transmitted from the signal processing circuit 307 to form an electrostatic latent image on the photosensitive drum 305. The electrostatic latent image is developed by a developing device and transferred onto a sheet or the like by a transfer device.
On the other hand, a laser beam emitted backward (to the left in FIG. 6) from the semiconductor laser diode 301 falls on a photodetector 309 which is composed of a photo diode, and the photodetector 309 detects the intensity of the laser beam. Then, a PD monitor section 310 as a control circuit controls the semiconductor laser control circuit 308 according to an output signal from the photodetector 309 such that the intensity of light outputted from the semiconductor laser diode 301 becomes equal to a target value. On this occasion, the PD monitor section 310 provides control such that values of current for driving respective light-emitting elements of the semiconductor laser diode 301 are regulated according to external signals corresponding to the respective light-emitting elements so that the intensities of light beams outputted from the respective light-emitting elements become equal to a fixed value, and are held at the regulated values.
FIG. 7 is a circuit diagram showing the detailed configuration of the laser control apparatus 300 in FIG. 6, and the laser control apparatus 300 is comprised of a control system that provides APC (Auto Power Control) control using sample-hold control, which is one of conventional methods for controlling laser light.
As shown in FIG. 7, the laser control apparatus is comprised of a laser driving section 211 that drives the semiconductor laser diode 301, a resistance 202 that limits current to be supplied to the semiconductor laser diode 301, an APC control section 212 that controls the intensity of light emitted from the semiconductor laser diode 301, a sample-hold capacitor 203 that determines electric charge to be accumulated when laser light is emitted during the APC control, the PD monitor section 310 that detects current flowing through the photodetector 309 composed of a photodiode in terms of voltage converted from current by a resistor 206, and a band gap 217 that supplies a reference voltage for current-to-voltage conversion of monitor current flowing through the photodiode 309 when laser light is emitted during the APC control.
With the laser control apparatus 300 constructed as above, the PD monitor section 310 causes the resistor 206 thereof to perform current-to-voltage conversion of the monitor current flowing through the photodiode 309 according to the intensity of light emitted from the semiconductor laser diode 301. The resulting voltage is then applied to an input terminal of a comparator 405 in the APC control section 212. Another input terminal of the comparator 405 is connected to the band gap 217 such that the voltage outputted from the PD monitor section 310 is compared with the reference voltage supplied from the band gap 217.
An output terminal of the comparator 405 is connected to an input terminal of a voltage controller 407 including a feedback loop such that voltage to be applied to the laser driving section 211 is controlled. According to the voltage thus controlled, the laser driving section 211 causes the semiconductor laser diode 301 to emit laser light. In this way, the intensity of laser light emitted from the semiconductor laser diode 301 is controlled.
To control the intensity of the emitted light in this way enables adjustment of the laser light intensity within a short period of time, and thus, the APC can be provided during the scanning of the marginal regions in one scanning where the laser beam scans the photosensitive drum 305 in the axial direction thereof (so-called inter-line APC).
In the above described conventional laser control apparatus that controls the intensity of laser light according to a voltage value obtained by current-to-voltage conversion of the monitor current flowing through the photodiode 309 and the reference voltage supplied from the band gap 217 during the APC, to vary the laser light intensity to be set, a method can be envisaged in which the setting of the reference voltage is changed. For example, a circuit implementing this method is configured such that resistances are connected in parallel with a circuit that sets the reference voltage, to be selectively turned on/off by a switch. However, this configuration has the problem that the circuit size and the cost increase.
Further, to variably control the intensity of laser light, a method has been proposed in which a voltage value obtained by current-to-voltage conversion of the monitor current is analog-to-digital converted, and the resulting voltage is compared with a predetermined reference voltage in terms of digital values (refer to Japanese Laid-Open Patent Publication (Kokai) No. 10-335732). According to this method, although it is possible to freely set the laser light intensity, it takes much time to adjust the laser light intensity. For this reason, a high-speed printer, for example, cannot provide the APC during the scanning of the marginal regions in one scanning where the laser beam scans the photosensitive drum 305 in the axial direction thereof, and thus the APC needs to be provided during printing of each page (so-called inter-page APC).